The present invention relates to dual servo systems and, in particular, to systems employing self-checking and independent checking of system operation.
A dual servo system has the advantage of tending to fail passively. In particular, a failed servo attempting to move incorrectly is opposed by the other servo. Consequently, the two servos are deadlocked and produce little net torque. It is known to install dual servos in an aircraft to actuate a control surface. This known system typically employs three independent dual analog control channels to control three classes of control surfaces: aileron, elevator and rudder. It has also been proposed to substitute a single digital computer for the three analog control channels. A serious drawback, however, is that a single digital computer may exhibit a generic software error or a failure mode whereunder all three control surfaces are driven to an extreme position. This type of failure is referred to herein as "multi-axis hardovers".
Therefore, there is a need for a dual servo system which may employ digital processing techniques but which is inherently reliable and avoids "multi-axis hardovers".